1. Field of the Invention
The present invention relates to an adaptive modulation method which can be applied to a mobile communications system, and more particularly to an adaptive modulation method for changing a modulation level based on a channel power gain.
2. Description of the Related Art
In digital communications, particularly in mobile communications systems, it is widely known that channel quality (such as a bit error rate) significantly degrades by variations in the received signal-to-noise ratio (SNR) due to fading. This phenomenon occurs when signal waves are affected by delayed signal waves including scattered signal waves. That is, the amplitude and the phase of a signal varies with time. The fading varies occur in the range of several tens of dBs.
The modulation method which is not adaptive to a fading channel (hereinafter referred to as a non-adaptive modulation method) is a method with which a modulation level (and a transmission power) is fixed. If this modulation method is applied, a fairly large link margin must be secured in order to maintain the channel quality allowed by the system in a time period during which the signal level drops due to fading (deep fades). To secure the link margin, for example, a strong error correction which trades off a transmission bandwidth must be applied. Namely, the system to which the non-adaptive modulation method is applied is effectively designed for the worst case channel conditions. Accordingly, with such a system, the channel capacity over fading channels cannot be efficiently utilized.
In recent years, an adaptive modulation method intended to efficiently utilize the capacity over a fading channel has been actively studied and developed.
The bit error rate performance of a modulation method depends on the distance between signal points implemented with the modulation method, that is, on Eb/No (or Es/No). This is common to any modulation methods.
Also an error correction technique intended to improve channel quality is a technique for regularly extending the distance between signal points by using redundant bits.
In fading channels, the probability of occurrence of the worst conditions is considerably small. That is, communications are made under the condition better than the maximum bit error rate allowed by the system in most of the total time. Accordingly, an optimum transmission can be realized by performing control so that the required bit error rate is maintained with an adaptive change of a transmission power level, a transmission symbol rate, a modulation level, a coding rate, or their combination, depending on a channel condition. This is the principle of an adaptive modulation method.
Adaptive modulation techniques provide high average spectral effectively by transmitting at high data rates with higher modulation levels under favorable channel conditions, and reducing throughput via lower modulation levels or transmission-off as the channel degrades.
The measurement of the channel condition can be performed by the instantaneous received SNR and the average received SNR.
FIG. 1A shows the principle of the adaptive modulation method. In this figure, x(i) is a transmission signal output from a transmitter 1501 at a station A which is a first station, y(i) is a received signal received by a receiver 1503 at a station B which is a second station opposing the station A, g(i) is a time-varying gain due to fading, and n(i) is an additive white Gaussian noise (AWGN).
For a bidirectional communications system, two sets of the system shown in FIG. 1A, which has reverse transmission directions, are included.
If the channel 1502 is a channel on which the fading of the link in the first direction (from the station A to the station B) correlates with that of the link in the second direction (from the station B to the station A), for example, a TDD (Time Division Duplex) channel, the following control operations are performed.
A channel estimator 1506 within the receiver 1503 at the station B on the first direction link estimates the power gain of the channel 1502, and notifies of the estimated channel information a demodulator/decoder 1507 within the receiver 1503 at the station B. The demodulator/decoder 1507 demodulates/decodes the received signal y(i) received from the first direction link after equalization based on the estimated channel information. Furthermore, the channel estimator 1506 within the receiver 1503 at the station B notifies an adaptive modulator/encoder 1504 within the transmitter 1501 at the station B on the second direction link, of the estimated channel information (or the estimated information obtained by applying an extrapolation-interpolation to the estimated power gain). The adaptive modulator/encoder 1504 sets the modulation level information according to the notified estimated channel information, and sends the transmission signal x(i) along with the modulation level information to the second direction link on the channel 1502.
The channel estimator 1506 within the receiver 1503 at the station A on the second direction link estimates the power gain of the channel 1502, and notifies of the estimated channel information the demodulator/decoder 1507 within the receiver 1503 at the station A. The demodulator/decoder 1507 demodulates/decodes the received signal y(i) received from the second direction link after equalization based on the estimated channel information. Additionally, the channel estimator 1506 within the receiver 1503 at the station A notifies the adaptive modulator/encoder 1504 within the transmitter 1501 at the station A on the first direction link, of the estimated channel information (or the estimated information obtained by applying an extrapolation-interpolation to the estimated power gain). The adaptive modulator/encoder 1504 sets the modulation level information according to the notified estimated channel information, and sends the transmission signal x(i) along with the modulation level information to the first direction link on the channel 1502.
In this way, a reciprocating transmission of the modulation level information can be implemented.
In the meantime, if the channel 1502 is a channel on which the fading of the first direction link does not correlate with that of the second direction link, for example, an FDD (Frequency Division Duplex) channel, the following control operations are performed.
First of all, the channel estimator 1506 within the receiver 1503 at the station B on the first direction link estimates the power gain of the channel 1502, and notifies of the estimated channel information the demodulator/decoder 1507 within the receiver 1503 at the station B. The demodulator/decoder 1507 demodulates/decodes the received signal y(i) received from the first direction link after equalization based on the estimated channel information. Additionally, the channel estimator 1506 within the receiver 1503 at the station B feeds back its estimated channel information (or the estimated information obtained by applying an extrapolation-interpolation to the estimated power gain) to the adaptive modulator/encoder 1504 within the transmitter 1501 (shown in FIG. 1A) at the station A on the first direction link by using a feedback channel 1508 for the first direction link. The adaptive modulator/encoder 1504 sets the fed-back modulation level information, and sends the transmission signal x(i) along with the modulation level information to the first direction link on the channel 1502.
Also the second direction link requires exactly the same feedback mechanism as that described above.
In FIG. 1A, a power controlling unit 1505 within the transmitter 1501 implements the above described power adaptation process.
FIG. 1B exemplifies the signal point arrangements implemented with respective modulation methods which can be selected by the adaptive modulator/encoder 1504 within the transmitter 1501 and the demodulator/decoder 1507 within the receiver 1503. As the modulation methods, QPSK (Quadri-Phase Shift Keying), 16 QAM (16 Quadrature Amplitude Modulation), 64 QAM, etc. can be selected.
Since the adaptive modulation method requires the processing unit for adaptively controlling a modulation method as described above, it has the trade-off between the performance and the complexity unlike the non-adaptive modulation method.
With the above described conventional adaptive modulation method, the modulation level information set by the adaptive modulator/encoder 1504 within the transmitter 1501 must be added as a control signal on the transmission signal x(i) sent by the transmitter 1501, as stated before. Therefore, the transmission efficiency degrades.
This control signal must be sent every state change period (such as every normalized maximum fading frequency). Because an error of the control signal causes the entire received information for one period (one block) to be lost, the error rate of the control signal must be decreased to a fairly low level. Accordingly, the conventional adaptive modulation method requires also the redundancy for correcting an error of the control signal.
Up to now, also the method for preventing the transmission efficiency from decreasing by embedding the modulation level information in the control signal (such as a preamble) used for another purpose, and (not by estimating but) by demodulating the control signal on a receiving side has been proposed. This method, however, imposes a restriction on the pattern of the control signal, which leads to a lack of generality and universality.
The present invention was developed in the above described background, and aims at preventing the transmission efficiency of a signal from decreasing by allowing the maximum likelihood estimation of a modulation level to be made on a receiving side without transmitting any control signal from a transmitting side, particularly in an adaptive modulation method for changing a modulation level based on a channel power gain.
The present invention assumes a radio transmission technology for adaptively changing a modulation level according to the state of a transmission path.
A first aspect of the present invention has the following configuration.
First of all, the mean or the expectation value of a carrier wave power is calculated for received signals to which no modulation level information is added when being sent.
Next, the difference between the calculated mean or expectation of the carrier wave power of received signals, and a mean or an expectation of the carrier wave power, which is prescribed for each modulation level, is calculated as the likelihood of each modulation level.
Then, the modulation level corresponding to the maximum likelihood value among the likelihood values of respective modulation levels is estimated as the modulation level of the received signals.
With the configuration according to the first aspect of the present invention, the differences between average CNRs of respective modulation levels when a bit error rate (that is, an instantaneous Es/No) is made constant, are considered and the likelihood values based on these differences are used, so that the maximum likelihood estimation of a transmitted modulation level can be made only from received signals without obtaining the conventionally required modulation level information when the signals are transmitted, in a radio transmission technology for adaptively changing the state of a transmission path within a system expected to be operated under a fading environment such as a mobile communications environment, etc.
Namely, with the above described configuration according to the present invention, the absolute value of a likelihood value, that is, the dynamic range of the likelihood value can be reduced by performing a likelihood calculation for each received symbol, thereby saving computer resources in an actual apparatus.
A second aspect of the present invention has the following configuration.
First of all, the variance of a carrier wave power is calculated for received signals to which no modulation level information is added when being sent.
Then, the difference between the calculated variance of the carrier wave power of received signals and a variance of the carrier wave power, which is prescribed for each modulation level, is calculated to be a likelihood of each modulation level.
Then, the modulation level corresponding to the maximum likelihood value among the likelihood values of respective modulation levels is estimated as the modulation level of the received signals.
With the above described configuration according to the second aspect of the present invention, the maximum likelihood estimation of a modulation level can be made with a higher accuracy only from received signals by considering the differences between the variances of signal points at respective modulation levels, and by using the likelihood values based on these differences.
A third aspect of the present invention has the following configuration.
First of all, the sample variance of the carrier wave power is calculated for signals to which no modulation level information is added when being sent.
Next, the difference between the calculated sample variance of the carrier wave power of received signals and a sample variance of the carrier wave power, which is prescribed for each modulation level, is calculated to be the likelihood of each modulation level.
Then, the modulation level corresponding to the maximum likelihood value among the likelihood values of respective modulation levels is estimated as the modulation level of the received signals.
With the above described configuration according to the third aspect of the present invention, the maximum likelihood estimation of a modulation level can be made with a higher accuracy only from received signals by using a likelihood value based on the distance between the sample variance of a signal point of received signals, and the sample variance at each modulation level.
A fourth aspect of the present invention has the following configuration.
The variance of a carrier wave power is calculated for the received signals to which no modulation level information is added when being transmitted.
Next, the difference between the calculated variance of the carrier wave power of received signals and a sample variance of the carrier wave power, which is prescribed for each modulation level, is calculated to be the likelihood of each modulation level.
Then, the modulation level corresponding to the maximum likelihood value among the likelihood values of respective modulation levels is estimated as the modulation level of the received signals.
With the above described configuration according to the fourth aspect of the present invention, the maximum likelihood estimation of a modulation level can be made with a higher accuracy and a higher operation efficiency only from received signals by using the likelihood value based on the distance between the variance of a signal point of received signals and the sample variance at each modulation level.
In the configurations of the present invention described so far, an estimation block length of a signal sent at a same modulation level can be set according to an estimation error rate allowed by a system. This also means that if a block length is set based on a fading cycle, the estimated number of symbols (block length) can be set to an arbitrary value which is equal to or smaller than the estimated number.
As a result, since a period (such as a normalized maximum fading frequency) for which a modulation level is desired to be changed becomes longer particularly in a moderate or high-speed system, the estimation of a modulation level can be made with a higher accuracy by estimating the modulation level on the entire period. Additionally, an estimation block length can be shortened without changing an allowed estimation error rate in this case, which leads to the reduction in the complexity.
In the configurations of the present invention described so far, a received signal level is obtained, so that the modulation level range in which a likelihood value is calculated can be restricted based on the obtained received signal level.
Consequently, the amount of the entire estimation calculation process can be reduced.
In the configurations of the present invention described so far, the modulation level range in which a likelihood value is calculated can be restricted when a current estimation is made, based on the modulation level which is most recently estimated.
As a result, the amount of the entire estimation calculation process can be reduced.
In the configurations of the present invention described so far, a subsequent estimation operation of a likelihood value at a corresponding modulation level can be suspended if the information about the distance between signal points, which is calculated by a different maximum likelihood value determination algorithm, exceeds a certain threshold value.
Consequently, the amount of the entire estimation process can be reduced.
In the configurations of the present invention described so far, a non-adaptive modulation method can be implemented by fixedly setting a modulation level at the time of a transmission.
In consequence, the real coexistence of an adaptive and a non-adaptive modulation method can be realized in a system where, for example, the adaptive modulation method is employed only in one direction although both of the methods coexist, or the maximum commonality can be provided to both a base station and a terminal in a system where the adaptive modulation method is employed in both directions.